Golf balls have conventionally been constructed as either two piece balls or three piece balls. The choice of construction between two and three piece affects the playing characteristics of the golf balls. The differences in playing characteristics resulting from these different types of constructions can be quite significant.
Three piece golf balls, which are also known as wound balls, are typically constructed from a liquid or solid center surrounded by tensioned elastomeric material. Wound balls are generally thought of as performance golf balls and have a good resiliency, spin characteristics and feel when struck by a golf club. However, wound balls are generally difficult to manufacture when compared to solid golf balls.
Two piece balls, which are also known as solid core golf balls, include a single, solid core and a cover surrounding the core. The single solid core is typically constructed of a crosslinked rubber, which is encased by a cover material. For example, the solid core can be made of polybutadiene which is chemically crosslinked with zinc diacrylate or other comparable crosslinking agents. The cover protects the solid core and is typically a tough, cut-proof material such as SURLYN®, which is a trademark for an ionomer resin produced by DuPont. This combination of solid core and cover materials provides a golf ball that is virtually indestructible by golfers. Typical materials used in these two piece golf balls have a flexural modulus of greater than about 40,000 psi. In addition, this combination of solid core and cover produces a golf ball having a high initial velocity, which results in improved distance. Therefore, two piece golf balls are popular with recreational golfers because these balls provide high durability and maximum distance.
The stiffness and rigidity that provide the durability and improved distance, however, also produce a relatively low spin rate in these two piece golf balls. Low spin rates make golf balls difficult to control, especially on shorter shots such as approach shots to greens. Higher spin rates, although allowing a more skilled player to maximize control of the golf ball on the short approach shots, adversely affect driving distance for less skilled players. For example, slicing and hooking the ball are constant obstacles for the lower skill level players. Slicing and hooking result when an unintentional side spin is imparted on the ball as a result of not striking the ball squarely with the face of the golf club. In addition to limiting the distance that the golf ball will travel, unintentional side spin reduces a player's control over the ball. Lowering the spin rate of the golf ball reduces the adverse effects of unintentional side spin. Hence, recreational players typically prefer golf balls that exhibit low spin rate.
Various approaches have been taken to strike a balance between the spin rate and the playing characteristics of golf balls. For example, additional core layers, such as intermediate core and cover layers are added to the solid core golf balls in an attempt to improve the playing characteristics of the ball. These multi-layer solid core balls include multi-layer core constructions, multi-layer cover constructions and combinations thereof. In a golf ball with a multi-layer core, the principal source of resiliency is the multi-layer core. In a golf ball with a multi-layer cover and single-layer core, the principal source of resiliency is the single-layer core.
In addition, varying the materials, density or specific gravity among the multiple layers of the golf ball controls the spin rate. In general, the total weight of a golf ball has to conform to weight limits set by the United States Golf Association (“USGA”). Although the total weight of the golf ball is controlled, the distribution of weight within the ball can vary. Redistributing the weight or mass of the golf ball either toward the center of the ball or toward the outer surface of the ball changes the dynamic characteristics of the ball at impact and in flight. Specifically, if the density is shifted or redistributed toward the center of the ball, the moment of inertia of the golf ball is reduced, and the initial spin rate of the ball as it leaves the golf club increases as a result of the higher resistance from the golf ball's moment of inertia. Conversely, if the density is shifted or redistributed toward the outer surface of the ball, the moment of inertia is increased, and the initial spin rate of the ball as it leaves the golf club would decrease as a result of the higher resistance from the golf ball's moment of inertia.
The redistribution of weight within the golf ball is typically accomplished by adding fillers to one or more of the core or cover layers of the golf ball. Conventional fillers include the high specific gravity fillers, such as metal or metal alloy powders, metal oxide, metal stearates, particulates, and carbonaceous materials and low specific gravity fillers, such as hollow spheres, microspheres and foamed particles. However, the addition of fillers may adversely interfere with the resiliency of the polymers used in golf balls and thereby the coefficient of restitution of the golf balls.
Prior art golf balls have multiple core layers to provide desired playing characteristics. For example, U.S. Pat. No. 5,184,828 claims to provide a golf ball having two core layers configured to provide superior rebound characteristics and carry distance, while maintaining adequate spin rate. More particularly, the patent teaches an inner core and an outer layer and controlling the hardness distribution in the outer layer and in the inner core in such a way that the golf ball has a maximum hardness at the outer site of the inner core. The patent alleges that such a distribution of hardness in the core assembly allows high energy to accumulate at the interface region where the hardness is at a maximum. The patent further claims that the energy of the club face is efficiently delivered to the maximum hardness region and transferred toward the inner core, resulting in a high rebound coefficient. However, since golf balls having hard cores and soft covers provide the most spin, the distribution taught by this patent would result in maximum core hardness at the interface when hit by a driver. Therein the ball has a relatively high driver spin rate and not very good distance. Since the ball in this patent has a softer outer core layer, the ball should have a lower spin rate for shorter shots such as an eight iron, where spin is more desirable. Thus, the ball taught by this patent appears to have many disadvantages.
U.S. Pat. No. 6,786,838 of Sullivan et al. discloses golf balls having at least three core layers (and up to six core layers) wherein the thickness of each core layer is at least twice as thick as an adjacent outer core layer and each core layer having a different hardness. The core layers have either progressively increasing or decreasing hardness from the innermost core layer to the outermost core layer.
However, none of these references discloses a multi-layered core golf ball wherein each core layer has a plurality of hardnesses and a hardness gradient (positive, negative or a combination) within each respective core layer in addition to a hardness gradient as between core layers.
Co-pending related U.S. patent application Ser. Nos. 12/469,258, 12/469,312, 12/492,514 and 12/492, 570, incorporated herein by reference, disclose and claim golf balls having single layer cores comprising different regions of varying hardness within the single layer core. The present invention extends this to the multi-layer core golf ball in order to reduce or eliminate the increased manufacturing costs and difficulty which often result when the properties of inner core layers are undesirably altered or deteriorated as outer core layers are cured or otherwise mounted or formed around the inner core layer by applying heat. The inventive plurality of hardnesses and hardness gradient within each layer of the multi-layered golf balls of the present invention therefore provide and optimize all of the benefits of a multi-layer core golf ball meanwhile reducing and minimizing the number of core layers heretofore necessary in order to achieve and optimize those benefits.